/Engine/Source/Editor/ViewportInteraction/Public/ViewportWorldInteraction.h
When using VR, this sets the viewport client that's been "possessed" by the head mounted display. Only valid when VR is enabled.
void SetDefaultOptionalViewportClient ( const [TSharedPtr](API\Runtime\Core\Templates\TSharedPtr)< class [FEditorViewportClient](API\Editor\UnrealEd\FEditorViewportClient) > & InEditorViewportClient )